An investigation has been ordered into the suicide of a Russian opposition activist who fled to the Netherlands seeking political asylum.

Aleksandr Dolmatov took his own life on Thursday, the day after he was detained in an immigration detention centre in Rotterdam.

His lawyer, Marq Wijngaarden, said the 36-year-old should not have been held there while his appeal case was ongoing.

Dolmatov left a suicide note, addressed to his mother, in which he described himself as having “betrayed the security of the motherland”.

A spokesman for under-secretary for justice, Fred Teeven, said the letter indicated he was troubled by “tragic but personal issues” and did not go into his reasons for fleeing to the Netherlands.

But Wijngaarden said it was possible that his client had been put under pressure by Russian security services.

Teeven has ordered a full investigation into the circumstances of Dolmatov’s death, a call which has been echoed by the Russian foreign ministry.

Dolmatov was a leader of the opposition movement Another Russia. He fled his homeland in June, a month after taking part in a protest against Vladimir Putin’s re-election.

He worked as an engineer for a munitions firm that designs tactical rockets for the Russian arms industry. In his last months in Russia he claimed to have come under increasing pressure from the FSB to cease his political activities.

The day after he arrived in the Netherlands police searched his parents’ house as part of an operation targeting anti-government activists.

His claim for asylum was turned down in November after the Dutch Immigration and Naturalisation Service (IND) found the worst sanction he could expect to face for his part in the protest was a 500-rouble fine (€12.50).

Wijngaarden said that a few days after that decision he noticed a drastic change in his client’s character.

Dolmatov suddenly became fearful and banned his lawyer from discussing Russian agents or even the names of FSB agents.

“He said this was ‘progressive insight’, but I didn’t find his explanation credible,” Wijngaarden told NRC.

“I told him that making threats is a crime in the Netherlands. But he denied, as he stirred his teabag, that he had been threatened.”

Dolmatov was placed in the detention centre in Rotterdam on Wednesday following a previous suicide attempt at the weekend.

Wijngaarden said the immigration service acted beyond its powers in detaining his client while his appeal against the refusal to grant him asylum was still being considered.

The IND refused to comment on the claim that Dolmatov had been held in the centre unlawfully.